Conventionally, as a pneumatic tire having run-flat traveling performance, a side rubber reinforcement layer formed of a hard rubber with a crescent-shaped cross section is provided on a side wall portion, and a certain traveling distance is ensured based on the rigidity of this side rubber reinforcement layer, even if the air pressure drops in the event of a puncture. This side rubber reinforcement layer secures a high level of rigidity by suppressing heat build-up at the time of run-flat traveling, so the layer is required to have low heat build-up and high rigidity.
As concern over global environmental issues increases, there is an increased demand for the improvement of fuel consumption performance during normal travel. In order to improve fuel consumption performance, it is necessary to suppress heat build-up at the time of traveling and to reduce the rolling resistance of tires. Therefore, heat build-up in the rubber composition constituting pneumatic tires has been suppressed, and the rolling resistance when a tire is produced has been made smaller. As an indicator of the heat build-up in a rubber composition, tan δ at 60° C. determined by dynamic visco-elasticity measurement is typically used, and smaller tan δ (60° C.) of the rubber composition indicates smaller heat build-up.
As a method of reducing the tan δ (60° C.) of a rubber composition, Japanese Unexamined Patent Application Publication No. 2010-132872A proposes adding silica and a silane coupling agent to a rubber component containing modified butadiene. However, in recent years there is a strong demand to further reduce the rolling resistance and to achieve a high level balance between low rolling resistance and ride comfort and steering stability during normal travel, and there is a demand to improve the performance described above from the perspective of not only a silica-containing system, but also carbon black.
Examples of methods of reducing the tan δ (60° C.) of a rubber composition with carbon black include reducing the compounding ratio of carbon black, increasing the specific surface area of carbon black, and reducing the size of aggregates. However, such methods are problematic in that the mechanical characteristics such as tensile break strength and the rubber hardness are diminished and that the ride comfort and/or steering stability is diminished when a tire is produced.
Therefore, Japanese Unexamined Patent Application Publication No. 2004-519552A proposes reducing heat build-up in a rubber composition primarily by adding carbon black having an adjusted specific surface area (BET (Brunauer-Emmett-Teller) specific surface area, CTAB (cetyltrimethyl ammonium bromide) specific surface area, and iodine adsorption index (IA)), DBP (dibutyl phthalate) structure value, Stokes diameter (Dst), and the like. However, in the case of this rubber composition, the effect of combining both low heat build-up in the rubber composition and ride comfort and steering stability during normal travel is not necessarily sufficient, and there has been a demand for further improvements.